An active matrix substrate used in a liquid crystal display device, or the like, includes a plurality of gate bus lines (gate lines) extending in the row direction, and a plurality of source bus lines (source lines) extending in the column direction. A switching element such as a thin film transistor (hereinafter “TFT”) and a pixel electrode are arranged in each pixel defined by these lines. In recent years, it has been proposed in the art to use, as the switching element, a TFT (hereinafter referred to as an “oxide semiconductor TFT”) using an oxide semiconductor layer as the active layer.
Source lines and source and drain electrodes of TFTs are normally formed in the same layer (hereinafter, referred to as a “source wiring layer”). Similarly, gate lines and gate electrodes of TFTs are formed in the same layer (hereinafter, referred to as a “gate wiring layer”). A gate wiring layer may include CS bus lines, CS electrodes, etc. Note that a “wiring layer” as used in the present specification includes a plurality of conductive layers such as electrodes and lines formed by using a common conductive film. Among others, a wiring layer formed by using a metal film is specifically referred to as a “metal wiring layer”.
A gate wiring layer and a source wiring layer are typically formed by using a metal material having a low resistance, and the surface thereof has a metallic luster. Therefore, light (visible light) coming from outside to be incident upon the substrate may possibly be reflected at the surface of these wiring layers (hereinafter, referred to as “ambient light reflection”), thereby lowering the display contrast.
In order to address this, according to Patent Document 1, for example, for the purpose of suppressing the reflection at the gate wiring layer, a discolored layer having a smaller reflectance than an Al alloy is formed, on the upper surface of the gate wiring layer made of an Al—Ni alloy, using a discoloration solution ((CH3)4NOH and H2O).
On the other hand, in order to accommodate larger screen sizes and higher definitions of liquid crystal display devices, there has been a demand for further reducing the resistance of the lines and electrodes of the active matrix substrate. In view of this, it has been proposed to use Cu (copper), a Cu alloy, or the like, having a lower electric resistance than Al, for the source wiring layer and the gate wiring layer (e.g., Patent Document 2).